Methods for manufacturing high chromium (Cr) molten steel, methods for manufacturing cast steel, and methods for manufacturing rolling rolls

Abstract

A method for producing high chromium (Cr) molten steel is provided that can produce high chromium (Cr) molten steel without polluting the converter when producing high chromium (Cr) molten steel using a converter. [Solution] The present invention relates to a method for producing high chromium (Cr) molten steel having a chromium (Cr) content of 4.5 wt% to 5.5 wt%, and includes the steps of charging molten steel into a converter used in a stainless steelmaking process, and charging a chromium ferroalloy containing chromium (Cr) into the converter so that the chromium (Cr) content in the molten steel is 4.5 wt% to 5.5 wt%, the oxygen blowing process of blowing oxygen into the converter into which chromium alloy ferroalloy is charged to remove carbon (C) from the molten steel, the process of adding a deoxidizer to the converter to remove oxygen (O) from the molten steel, and the process of adding a reducing agent to the converter to reduce chromium oxide contained in slag floating on the surface of the molten steel to chromium (Cr), and the steelmaking process further comprises the steps of tapping the molten steel from the converter into a ladle, and a temperature control process of heating the molten steel received in the ladle using a ladle refining furnace (LF), which is a heating device used in a carbon steelmaking process, and a degassing process of removing hydrogen (H2) and nitrogen (N2) from the molten steel received in the ladle using a reflux vacuum degassing (RH) device, which is a vacuum drawing device used in a steelmaking process of carbon steel, The degassing process is performed after the temperature adjustment process is completed.

Description

[Technical Field]

[0001] The present invention relates to a method for producing high chromium (Cr) molten steel, a method for producing a slab, and a method for producing a rolling roll, and more particularly to a method for producing high chromium (Cr) molten steel, a method for producing a slab, and a method for producing a rolling roll, which can efficiently produce high chromium (Cr) molten steel and can easily and inexpensively suppress or prevent the occurrence of segregation. [Background technology]

[0002] Rolling rolls are manufactured by solidifying molten steel containing a high chromium (Cr) content to produce an ingot, which is then forged. More specifically, molten steel containing a high chromium (Cr) content is first produced and solidified to produce an ingot. The ingot is then forged to produce a rolling roll. However, when the ingot is directly forged, segregation within the ingot can cause uneven structure of the roll, leading to cracks and low hardness. Therefore, before forging the ingot, the segregation within the ingot is removed using an electroslag remelting (ESR) method. In the ESR method, an electrode is first produced using the produced ingot. An arc is then generated using the produced electrode to remelt the electrode. The molten steel formed by the remelting of the electrode is then dropped into a water droplet and solidified to produce an ingot again. At this time, the molten steel formed by remelting is dripped in the form of water droplets, so that an ingot with little segregation can be produced.

[0003] However, the ESR method requires multiple steps, including the steps of manufacturing an electrode rod from the ingot produced as described above, remelting the electrode rod, and dripping the remelted molten steel to resolidify it. This results in significant time and cost involved in the process of suppressing segregation. [Prior art documents] [Patent documents]

[0004] [Patent Document 1] Korean Patent Registration No. 10-1346636 Summary of the Invention [Problem to be solved by the invention]

[0005] An object of the present invention is to provide a method for producing high chromium (Cr) molten steel, which can produce high chromium (Cr) molten steel without polluting the converter when producing high chromium (Cr) molten steel using a converter.

[0006] The present invention provides a method for manufacturing a slab and a method for manufacturing a rolling roll that can simplify the process of suppressing or preventing the occurrence of segregation and reduce costs.

[0007] The present invention provides a method for producing a slab and a method for producing a rolling roll that can suppress or prevent the occurrence of cracks, can make the structure uniform, and has high hardness. [Means for solving the problem]

[0008] The method for producing high chromium (Cr) molten steel of the present invention is a method for producing high chromium (Cr) molten steel having a chromium (Cr) content of 4.5 wt% to 5.5 wt%, and is characterized by comprising the steps of charging molten steel into a converter used in a stainless steelmaking process, and introducing a chromium ferroalloy containing chromium (Cr) into the converter so that the chromium (Cr) content in the molten steel is 4.5 wt% to 5.5 wt%.

[0009] The method for producing high chromium (Cr) molten steel of the present invention may include an oxygen blowing step of blowing oxygen into the converter into which a chromium ferroalloy has been charged to remove carbon (C) from the molten steel, a step of charging a deoxidizer into the converter to remove oxygen (O) from the molten steel, and a step of charging a reducing agent into the converter to reduce chromium oxide contained in slag floating on the surface of the molten steel to chromium (Cr).

[0010] The method for producing high chromium (Cr) molten steel of the present invention may include: a step of tapping the molten steel from the converter into a ladle; and a temperature adjustment step of heating the molten steel received in the ladle using a ladle refining furnace (LF), which is a heating device used in a carbon steelmaking process.

[0011] In the temperature adjustment process, the temperature of the molten steel is preferably adjusted to 1560°C to 1600°C.

[0012] The method for producing high chromium (Cr) molten steel of the present invention includes a degassing process for removing hydrogen (H) and nitrogen (N) from the molten steel received in the ladle using a reflux vacuum degassing (RH) device, which is a vacuum drawing device used in the steelmaking process of carbon steel, and the degassing process may be performed after the temperature adjustment process is completed.

[0013] When the molten steel from the converter is tapped into a ladle, it is preferable to tap into a ladle used in a carbon steel making process.

[0014] The method for producing a slab of the present invention is characterized by comprising: a step of preparing molten steel containing 4.5 wt% to 5.5 wt% chromium (Cr); a casting step of supplying the molten steel to a mold of a casting device and solidifying the molten steel inside the mold to produce a slab containing unsolidified molten steel; a step of withdrawing the slab produced in the casting step to below the mold; and a solidification step of applying a magnetic field to the slab withdrawn to below the mold to solidify the unsolidified molten steel contained in the slab while causing it to flow, thereby producing the slab.

[0015] The solidification step may include heating an end portion of the strand drawn below the mold.

[0016] The casting process may include applying a magnetic field to the mold to cause molten steel inside the mold to flow.

[0017] When the cast piece is withdrawn to the bottom of the mold, it is preferable to withdraw it at a speed of 0.04 m / min or less.

[0018] The solidification process includes a step of spraying cooling water onto the slab drawn below the mold, and it is preferable that the cooling water be sprayed so that the surface temperature of the slab reaches 800°C to 900°C.

[0019] It is preferable that in the drawing step, the slab is drawn from the mold so as to be perpendicular to the ground, and that in the solidification step, the slab is solidified in a state in which it is disposed so as to be perpendicular to the ground.

[0020] The method for manufacturing a rolling roll of the present invention includes the steps of heating a slab, forging the heated slab into the shape of a rolling roll, and heating the rolling roll manufactured in the forming step to remove hydrogen (H) from the rolling roll, and is characterized in that the step of heating the slab includes a step of increasing the temperature of the slab to a target temperature in multiple stages.

[0021] The process of heating the slab to the target temperature may include a process of heating the slab to a first temperature of 250°C to 350°C, a process of heating the slab to a second temperature of 450°C to 550°C, a process of heating the slab to a third temperature of 650°C to 750°C, and a process of heating the slab to the target temperature of 1100°C to 1250°C.

[0022] When heating the slab to the first to third temperatures and the target temperature, the slab may be held at the first temperature for 3 to 5 hours, the slab at the second temperature for 5 to 7 hours, the slab at the third temperature for 3 to 5 hours, and the slab at the target temperature for 14 to 18 hours.

[0023] When forging the slab, the slab heated to the temperature of 1100°C to 1250°C may be reduced and forged, and the thickness of the slab may be reduced by 250 mm to 350 mm each time the slab is reduced using a reduction device.

[0024] The step of removing hydrogen (H2) from the reduction roll may include a step of heating the reduction roll to a temperature of 200°C to 400°C.

[0025] When the rolling roll is heated to the temperature of 200° C. to 400° C., it is preferable to heat it for 48 hours or more. [Effects of the Invention]

[0026] According to the present invention, high chromium (Cr) molten steel can be produced using a converter used for making steel of other steel types without contaminating the converter used for making steel of other steel types. Furthermore, when raising the temperature of the high chromium (Cr) molten steel or performing degassing by venting gases, the temperature of the molten steel can be effectively raised, thereby improving degassing efficiency.

[0027] Furthermore, it is possible to suppress or prevent the occurrence of segregation inside a slab more simply and at lower cost than in the past. Furthermore, when forging a slab to manufacture a rolling roll, it is possible to suppress or prevent the occurrence of cracks, and to manufacture a rolling roll having a uniform structure and high hardness. [Brief explanation of the drawings]

[0028] [Figure 1] FIG. 1 is a flow chart showing a method for manufacturing a rolling roll of the present invention. [Figure 2] FIG. 2 is a process flow diagram showing the steps (a) to (g) of preparing molten steel in order according to the present invention. [Figure 3] 3A to 3D are diagrams showing the operation (a) to (d) of the casting device of the present invention in order. [Figure 4] FIG. 2 is a flow chart showing steps (a) to (c) in order of a method for manufacturing a rolling roll using a cast slab manufactured by a method according to an embodiment of the present invention. [Figure 5] (a) and (b) show the results of cutting the cross section of a rolling roll, etching the cross section, and checking whether or not coarse segregation occurs. DETAILED DESCRIPTION OF THE INVENTION

[0029] The present invention will be described in more detail below with reference to the accompanying drawings. However, the present invention is not limited to the following and may be embodied in various different forms, which are provided merely to complete the disclosure of the present invention and to fully convey the scope of the invention to those skilled in the art. The drawings may be exaggerated in order to explain the present invention, and the same reference numerals in the drawings indicate the same components.

[0030] FIG. 1 is a flow chart showing a method for manufacturing a rolling roll of the present invention.

[0031] As shown in FIG. 1, the method for manufacturing a rolling roll includes a step of preparing molten steel (S100), a step of solidifying the molten steel to produce a slab (S200), a step of heating the slab (S300), a step of forging the heated slab to produce a rolling roll (S400), and a step of heating the rolling roll (S500).

[0032] The rolling roll is an alloy ferroalloy containing a high amount of chromium (Cr). More specifically, the rolling roll is a high chromium (Cr) alloy ferroalloy containing a high amount of chromium (Cr) of 4.5 wt% to 5.5 wt% throughout the roll. More specifically, the rolling roll as a whole is an alloy iron containing 4.5 wt% to 5.5 wt% of chromium (Cr), 0.75 wt% to 0.95 wt% of carbon (C), 0.2 wt% to 0.5 wt% of silicon (Si), 0.2 wt% to 0.5 wt% of manganese (Mn), 0.4 wt% to 0.65 wt% of molybdenum (Mo), 0.5 wt% or less (0 wt% or more) of nickel (Ni), 0.2 wt% or less (0 wt% or more) of copper (Cu), 0.0025 wt% or less (0 wt% or more) of phosphorus (P), 0.015 wt% or less (0 wt% or more) of sulfur (S), and the remainder being iron (Fe) (91.16 wt% to 93.15 wt%).

[0033] The chromium (Cr) content is adjusted to 4.5 wt% to 5.5 wt% and the carbon (C) content is adjusted to 0.75 wt% to 0.95 wt% based on the entire roll in order to ensure hardness and workability. For example, if the chromium (Cr) content in the roll is less than 4.5 wt% or the carbon (C) content is less than 0.75 wt%, the hardness of the roll may be low. Conversely, if the chromium (Cr) content in the roll exceeds 5.5 wt% or the carbon (C) content exceeds 0.95 wt%, the workability of the roll may be low, making it difficult to process the manufactured roll as needed.

[0034] The reason for adjusting the phosphorus (P) content in the rolling roll to 0.025 wt% or less and the sulfur (S) content to 0.015 wt% or less is to make the roll structure uniform and suppress or prevent cracks. That is, if the phosphorus (P) content in the rolling roll exceeds 0.025 wt% or the sulfur (S) content exceeds 0.015 wt%, the roll structure may become non-uniform due to segregation inside the roll. As a result, when the roll is used to press the workpiece, cracks may occur in the roll. Therefore, the phosphorus (P) content in the rolling roll is adjusted to 0.025 wt% or less, and the sulfur (S) content is adjusted to 0.015 wt% or less.

[0035] The rolling roll to be manufactured in the present invention may be a roll to be used in a rolling mill that applies force to an object to be rolled, such as a cast slab or a steel plate, to roll it.

[0036] In the step of preparing molten steel (S100), molten steel for manufacturing a rolling roll is prepared. That is, in the step of preparing molten steel (S100), molten steel having the content of components that the rolling roll should have is prepared. That is, high chromium (Cr) molten steel having a chromium (Cr) content of 4.5 wt% to 5.5 wt% is prepared. More specifically, when preparing molten steel, molten steel containing 4.5 wt% to 5.5 wt% of chromium (Cr), 0.75 wt% to 0.95 wt% of carbon (C), 0.2 wt% to 0.5 wt% of silicon (Si), 0.2 wt% to 0.5 wt% of manganese (Mn), 0.4 wt% to 0.65 wt% of molybdenum (Mo), 0.5 wt% or less (0 wt% or more) of nickel (Ni), 0.2 wt% or less (0 wt% or more) of copper (Cu), 0.0025 wt% or less (0 wt% or more) of phosphorus (P), 0.015 wt% or less (0 wt% or more) of sulfur (S), and the balance being iron (Fe) (91.16 wt% to 93.15 wt%) is produced. This can be accomplished by sequentially performing the steps of the molten steel preparation process described below.

[0037] 2(a) to 2(g) are process flow diagrams showing the molten steel preparation process of the present invention in order.

[0038] As shown in FIG. 2 , the process of preparing molten steel M (S100) may include a process of treating the molten steel M using a converter 11 (hereinafter referred to as a converter treatment process (S110)), a process of tapping the molten steel M inside the converter 11 into a ladle 20 (S120), a temperature control process of adjusting the temperature of the molten steel M tapped into the ladle 20 using a temperature control device 30 (S130), and a degassing process of removing gas from the molten steel M using a vacuum drawing device 40 (S140).

[0039] The converter treatment step (S110) is a step of adjusting the composition of molten steel M while the molten steel M is charged into a converter 11. The converter 11 used in the converter treatment step (S110) is a converter used in a steelmaking process (hereinafter referred to as a stainless steel steelmaking process) for preparing molten steel for producing stainless steel (hereinafter referred to as molten steel for producing stainless steel). That is, when preparing the molten steel according to the embodiment, the converter 11 used in the stainless steel steelmaking process is utilized.

[0040] For a more detailed explanation, we will briefly explain the steelmaking process using a converter among the stainless steelmaking processes. The stainless steelmaking process includes a process of charging molten steel into a converter, a process of charging ferroalloys containing chromium (Cr) and ferroalloys containing nickel (Ni) into the converter, and an oxygen blowing process in which oxygen is injected into the converter using a lance to remove carbon (C), phosphorus (P), and other elements. When charging the ferroalloy into the converter, it is charged so that the chromium (Cr) content in the molten steel is 10.5 wt% to 11 wt%. Therefore, the molten steel prepared in the steelmaking process using a converter among the stainless steelmaking processes contains 10.5 wt% to 11 wt% chromium.

[0041] For comparison, a steelmaking process using a converter to prepare molten steel for producing carbon steel (hereinafter referred to as molten steel for producing carbon steel) (hereinafter referred to as the carbon steelmaking process) will be briefly described. The carbon steelmaking process includes a process of charging molten steel into a converter and an oxygen blowing process in which oxygen is injected into the converter using a lance to remove carbon (C), phosphorus (P), and the like. Carbon steel has a low chromium (Cr) content. Therefore, in the carbon steelmaking process, chromium (Cr) is not added to the converter, and the chromium (Cr) content in the molten steel is adjusted to a low level of 0.1 wt% to 1.0 wt%.

[0042] As described above, in a converter used in a stainless steelmaking process, molten steel with a high chromium (Cr) content of 10.5 wt% to 11 wt% is prepared. In other words, the converter used in the stainless steelmaking process contains molten steel with a high chromium (Cr) content of 10.5 wt% to 11 wt%. However, the converter used in the carbon steelmaking process contains molten steel with a low chromium (Cr) content of 0.1 wt% to 1.0 wt%.

[0043] Therefore, when a converter used in a carbon steelmaking process is used to prepare molten steel for manufacturing rolls having a high chromium (Cr) content of 4.5 wt% to 5.5 wt%, the converter may be contaminated. That is, when molten steel having a high chromium (Cr) content of 4.5 wt% to 5.5 wt% is poured into a converter for manufacturing carbon steel, a large amount of chromium (Cr) may remain inside the converter for manufacturing carbon steel even after the molten steel is tapped. For example, a large amount of chromium (Cr) may adhere or adhere to the inner wall of the converter for manufacturing carbon steel. As a result, the converter for manufacturing carbon steel may be contaminated by chromium (Cr). Furthermore, when a converter for manufacturing carbon steel with a large amount of residual chromium (Cr) is used in the carbon steelmaking process, a large amount of chromium (Cr) will be contained in the molten steel, and as a result, the chromium (Cr) acts as an impurity that degrades the quality of the carbon steel.

[0044] However, when a converter used in a stainless steelmaking process is used to prepare high-chromium (Cr) molten steel with a chromium (Cr) content of 4.5 wt% to 5.5 wt%, the converter for producing stainless steel does not become contaminated. That is, even if a large amount of chromium (Cr) remains inside the converter for producing stainless steel after molten steel with a chromium (Cr) content of 4.5 wt% to 5.5 wt% is poured into the converter for producing stainless steel and then tapped, the chromium remaining inside the converter does not act as an impurity. This is because the converter for producing stainless steel is a means for pouring molten steel with a high chromium content of 10.5 wt% to 11 wt%. Also, the chromium (Cr) content of the molten steel poured into the converter for producing stainless steel is even higher than that of the molten steel to be produced in the embodiment. Therefore, even if molten steel for producing rolling rolls is poured into the converter for producing stainless steel, the converter for producing stainless steel does not become contaminated. Therefore, when molten steel for producing stainless steel is produced again using a converter for producing stainless steel containing molten steel according to the embodiment, problems caused by chromium (Cr) in the molten steel previously contained therein do not occur.

[0045] Therefore, in the embodiment, when treating molten steel using a converter 11 (S110), a converter 11 used in the stainless steelmaking process as described above is used. Therefore, the converter 11 can be used without causing contamination of the converter 11 with chromium (Cr). Furthermore, a converter used to prepare molten steel according to the embodiment containing 4.5 wt% to 5.5 wt% chromium (Cr) can be used to prepare molten steel for stainless steel production. That is, a converter for stainless steel production can be used interchangeably in an operation to prepare molten steel for stainless steel production and an operation to prepare molten steel according to the embodiment containing 4.5 wt% to 5.5 wt% chromium (Cr).

[0046] The temperature adjustment device 30 used in the temperature adjustment process and the vacuum drawing device 40 used in the degassing process are a ladle refining furnace (LF) and a reflux vacuum degassing (RH) device used in a steelmaking process (hereinafter referred to as a carbon steel steelmaking process) for preparing molten steel for producing carbon steel (hereinafter referred to as molten steel for producing carbon steel). The reasons for using a ladle refining furnace and a RH device in a carbon steel steelmaking process will be described later.

[0047] The molten steel preparation process of the present invention will be described in more detail below with reference to FIG.

[0048] As described above, the molten steel preparation process (S100) includes a process (S110) of treating the molten steel M using the converter 11, a process (S120) of tapping the molten steel M from the converter 11 into a ladle, a process (S130) of adjusting the temperature of the molten steel M using the temperature control device 30, and a degassing process (S140) of removing gas from the molten steel M using the vacuum drawing device 40.

[0049] As shown in (a) to (d) of FIG. 2, the process (S110) of treating molten steel M using a converter 11 may include a process (S111) ((a) of FIG. 2) of charging a ferroalloy containing chromium (Cr) (hereinafter referred to as a ferroalloy chromium (Cr)) into the converter 11, an oxygen blowing process (S112) ((b) of FIG. 2) of injecting oxygen into the molten steel inside the converter 11, a deoxidation process (S113) ((c) of FIG. 2) of removing oxygen (O) contained in the molten steel M, and a process (S114) ((d) of FIG. 2) of reducing chromium oxide contained in slag floating on the surface of the molten steel M to chromium (Cr).

[0050] Although not shown, the molten steel preparation process may also include a pre-refining process for removing sulfur (S), phosphorus (P), and silicon (Si) from the molten steel before charging the molten steel into the converter. First, the pre-refining process will be briefly described. The pre-refining process may include a first pre-refining process for removing sulfur (S) from the molten steel and a second pre-refining process for removing phosphorus (P) and silicon (Si) from the molten steel. In the first pre-refining process, the sulfur (S) content in the molten steel is adjusted to 0.04 wt% or less. In the second pre-refining process, the silicon (Si) content in the molten steel is adjusted to 0.05 wt% or more and the phosphorus (P) content is adjusted to 0.03 wt% or less. In carrying out these first and second pre-refining processes, quicklime (CaO) and fluorite (CaF) are added to a ladle containing molten steel to remove sulfur (S), silicon (Si), and phosphorus (P) from the molten steel. The first and second pre-refining processes can be carried out in a hot metal pretreatment station (HMPS).

[0051] After the first and second pre-refining processes are completed, the molten steel M is charged into the converter 11. As described above, the converter 11 used in the stainless steel manufacturing process is used as the converter 11 into which the molten steel M is charged.

[0052] When molten steel M is charged into the converter 11, chromium (Cr) alloy ferroalloy is charged into the converter 11 as shown in FIG. 2(a). Therefore, the chromium (Cr) alloy ferroalloy charged into the converter 11 is melted by the heat of the molten steel M. At this time, the amount of chromium (Cr) alloy ferroalloy charged is adjusted so that the chromium (Cr) content of the entire molten steel M is 4.5 wt% to 5.5 wt%. As a result, molten steel M with a high chromium (Cr) content of 4.5 wt% to 5.5 wt% is produced.

[0053] Next, as shown in FIG. 2(b), oxygen blowing is performed, in which oxygen is blown or injected into the molten steel M inside the converter 11. That is, after a portion of a lance is inserted into the converter 11, oxygen (O) is supplied into the lance 12. For this purpose, oxygen (O) is blown or injected from the lance 12. The oxygen (O) blown into the converter 11 using the lance 12 reacts with carbon (O) contained in the molten steel M (C + O - → CO (gas)). At this time, the carbon (O) in the molten steel M reacts with oxygen (O) to form carbon monoxide (CO) gas, which is then exhausted outside the converter 11. As a result, the carbon (C) content in the molten steel M decreases. That is, decarburization occurs, in which the carbon (C) content of the molten steel M decreases. At this time, at least one of the flow rate and time of blowing oxygen is adjusted so that the carbon (C) content in the molten steel M is 0.75 wt% to 0.95 wt%.

[0054] When decarburization is promoted by blowing oxygen into the converter 11 in this way, reaction by-products consisting of metal oxides are generated in addition to carbon monoxide (CO) gas. The generated reaction by-products then rise to the upper surface of the molten steel M, i.e., rise to the surface side of the molten steel M, and float above the surface of the molten steel M. The reaction by-products floating on the surface of the molten steel in this way are slag (SL).

[0055] When the carbon (C) content in the molten steel M reaches 0.75 wt% to 0.95 wt% and decarburization is complete, or when the carbon (C) content in the molten steel M decreases and approaches 0.95 wt%, a dephosphorization agent is introduced into the converter 11. The dephosphorization agent is introduced while blowing oxygen into the converter 11 using the lance 12. The dephosphorization agent may be, for example, a material containing quicklime (CaO). As a result, phosphorus (P) in the molten steel M reacts with the dephosphorization agent and oxygen (O) (3CaO + 2P + 5O → 3CaO·P2O5). The reaction by-product, 3CaO·P2O5, then floats to the surface and is absorbed by the slag SL floating on the surface of the molten steel M. That is, the phosphorus (P) separated from the molten steel M is absorbed by the slag SL floating on the surface of the molten steel M. As a result, the phosphorus (P) content in the molten steel M decreases. That is, dephosphorization occurs, in which the phosphorus (P) content is reduced from the molten steel M. At this time, at least one of the amount of dephosphorization agent charged, the oxygen blowing flow rate, and the oxygen blowing time is adjusted so that the phosphorus (P) content in the molten steel M becomes 0.025 wt% or less.

[0056] After decarburization and dephosphorization are performed by blowing oxygen into molten steel M, the oxygen (O) content in molten steel M increases. The oxygen (O) in molten steel M causes pinholes in the cast slab. Furthermore, the oxygen (O) in molten steel reacts with the metals contained in the molten steel to form metal oxide inclusions. The inclusions in molten steel cause crack defects in the cast slab.

[0057] Therefore, after the oxygen blowing is completed, deoxidation is performed to remove oxygen (O) from the molten steel M. For this purpose, a deoxidizer containing at least one of silicon (Si) and aluminum (Al) is introduced into the converter 11. As the deoxidizer, at least one of an alloy containing silicon (Si) and an alloy containing aluminum (Al) can be used. In this case, the alloy containing silicon (Si) may be an Fe-Si ferroalloy containing silicon (Si) and iron (Fe), and may contain 70 wt% to 80 wt% of silicon (Si) and 20 wt% to 30 wt% of iron (Fe). Furthermore, the alloy containing aluminum (Al) may be an alloy close to pure aluminum (Al), containing 90 wt% to 95 wt% of aluminum (Al).

[0058] When a deoxidizer is added, it reacts with oxygen (O) contained in the molten steel M and at least one of silicon (Si) and aluminum (Al) contained in the deoxidizer. As a result, at least one of silicon oxide such as SiO2 and aluminum oxide such as Al2O3 is produced. The reaction by-product of at least one of silicon oxide and aluminum oxide is absorbed into slag SL floating on the surface of the molten steel M. As a result, the content of oxygen (O) contained in the molten steel M decreases (deoxidation). At this time, the amount of deoxidizer added is adjusted so that the content of oxygen (O) in the molten steel M is 0.001 wt% or less.

[0059] As explained above, at least one of an alloy containing silicon (Si) and an alloy containing aluminum (Al) is used as a deoxidizer. In this case, it is preferable to use a ferroalloy containing silicon (Si) and a ferroalloy containing aluminum (Al) in combination as a deoxidizer, compared to using an alloy containing silicon (Si) or an alloy containing aluminum (Al) as a deoxidizer.

[0060] On the other hand, if the oxygen (O) content in the molten steel exceeds 0.001 wt%, pinhole defects may occur in the cast slab due to the oxygen (O) and cracks may occur due to a large number of inclusions. Therefore, the oxygen (O) content in the molten steel M is adjusted to 0.001 wt% or less during the deoxidation process.

[0061] As described above, when oxygen blowing is performed to remove carbon (C) and phosphorus (P) from the molten steel M, the chromium (Cr) (S110) added to the molten steel M is oxidized and converted into chromium oxide. The generated chromium oxide is then absorbed by the slag SL floating on the surface of the molten steel M. This reduces the chromium (Cr) content in the molten steel. Therefore, the chromium oxide contained in the slag SL must be reduced to chromium and then resupplied to the molten steel M. For this purpose, a reducing agent is introduced into the converter 11 after deoxidation. More specifically, the reducing agent is introduced into the slag SL. The reducing agent may be a material containing silicon (Si), more specifically, a ferroalloy containing silicon (Si). More specifically, the reducing agent may contain 70 wt% to 80 wt% silicon (Si) and 20 wt% to 30 wt% iron (Fe).

[0062] When a reducing agent is introduced into the converter 11, the chromium oxide contained in the slag SL reacts with the silicon (Si) contained in the reducing agent. As a result, the chromium oxide contained in the slag SL is reduced to chromium (Cr), and the generated chromium (Cr) is absorbed or supplied to the molten steel. As a result, the chromium (Cr) content in the molten steel increases. At this time, the amount of reducing agent introduced is adjusted so that the chromium (Cr) content in the molten steel becomes 4.5 wt% to 5.5 wt%.

[0063] Once the reduction to chromium is complete, the component contents and temperature of the molten steel M are confirmed. For example, the molten steel M inside the converter 11 is sampled, and the component contents and temperature of the sampled molten steel are measured. Then, it is confirmed whether the measured component contents of the molten steel M are within the target component contents, and whether the measured temperature of the molten steel M is within the target temperature. Here, the target temperature may be, for example, 1650°C to 1750°C.

[0064] More specifically, the process checks whether the contents of chromium (Cr), carbon (C), phosphorus (P), sulfur (S), and oxygen (O) are within the target range. If the contents of chromium (Cr), carbon (C), phosphorus (P), sulfur (S), and oxygen (O) are within the target range and the temperature of the molten steel is within the target range, slag floating on the surface of the molten steel is removed (not shown). That is, the slag is removed from the surface of the molten steel. However, if the contents of chromium (Cr), carbon (C), phosphorus (P), sulfur (S), and oxygen (O) are not within the target range or if the temperature of the molten steel is not within the target range, the process for adjusting the content or temperature of the elements is performed again. That is, if the contents of chromium (Cr), carbon (C), phosphorus (P), sulfur (S), and oxygen (O) contained in the molten steel are not within the target contents, at least one of the processes of adding chromium (Cr) ferroalloy, decarburization, dephosphorization, desulfurization, deoxidation, and chromium reduction is performed. Also, if the temperature of the molten steel M is below 1650°C, the molten steel M is heated to raise the temperature of the molten steel to 1650°C or higher. Conversely, if the temperature of the molten steel exceeds 1750°C, the temperature of the molten steel is lowered.

[0065] On the other hand, if the content of each component contained in the molten steel deviates from the target content, there is a risk of a problem of a deterioration in the quality of the cast slab S. That is, if the content of at least one of chromium (Cr), carbon (C), phosphorus (P), sulfur (S), and oxygen (O) deviates from the target content, there is a risk of a problem of the hardness of the produced cast slab being weak or of pinholes and cracks occurring on the surface or inside of the cast slab. This may reduce the hardness of the rolling rolls and cause pinholes and cracks on the surface or inside of the rolls.

[0066] Furthermore, if the temperature of the molten steel M is below 1650°C, it may be difficult to raise the temperature of the molten steel M to the target temperature (the starting target temperature of the ladle refining furnace (LF)) in the subsequent temperature adjustment process. Therefore, when the molten steel M is supplied to the tundish, the temperature of the molten steel M may not reach the casting target temperature. In such a case, there is a risk of a problem in that the nozzle that supplies the molten steel from the tundish to the mold becomes clogged. Conversely, if the temperature of the molten steel M exceeds 1750°C, there is a risk of a problem in that the refractories that make up the converter may be eroded and damaged by the high heat. Therefore, the temperature of the molten steel M is adjusted to 1650°C to 1750°C in the converter treatment process.

[0067] When the content of each component in the molten steel M is within the target content and the temperature of the molten steel M is within the target temperature, slag floating on the surface of the molten steel M is removed (not shown).

[0068] After removing the slag SL from the converter 11, the molten steel M from the converter 11 is tapped into the ladle 20. At this time, it is preferable to tap the molten steel M into the ladle 20 used in the carbon steelmaking process. This is because the ladle 20 used in the carbon steelmaking process is used as the equipment used in the subsequent processes. That is, in the temperature adjustment process, a ladle refining furnace (LF) is used as the temperature adjustment device 30, and in the degassing process, a reflux vacuum degassing (RH) device is used as the evacuation device 40. At this time, the ladle refining furnace (LF) and the RH device are each equipment used in the carbon steelmaking process and are equipment connected to a ladle used in the carbon steelmaking process.

[0069] Furthermore, a ladle refining furnace (LF) is not used in the stainless steelmaking process. Also, a vacuum drawing device is used in the stainless steelmaking process, but this is a vacuum tank degasser (VTD), which is different from the RH device used in the carbon steelmaking process.

[0070] In this embodiment, when the molten steel inside the converter 11 is tapped into a ladle, the molten steel is tapped into the ladle 20 used in the carbon steel making process. That is, the molten steel is tapped into the ladle 20 used in the carbon steel making process in order to use the equipment used in the carbon steel making process in the subsequent temperature adjustment process and degassing process.

[0071] As described above, in the embodiment, a ladle refining furnace is used as the temperature control device 30, and an RH device is used as the evacuation device 40. Therefore, for ease of explanation, the temperature control device and the ladle refining furnace will be denoted by the same reference numeral "30" in the following description, and the evacuation device and the RH device will be denoted by the same reference numeral "40" in the following description.

[0072] After the molten steel M has been tapped into the ladle 20, the ladle 20 containing the molten steel M is moved to a temperature control device 30 and coupled to the temperature control device 30, as shown in FIG. 2(f). That is, the ladle 20 is moved to a ladle refining furnace 30, which is a temperature control device 30 used in the carbon steelmaking process, and coupled to the ladle refining furnace 30. As shown in FIG. 2(f), the ladle refining furnace may include a cover 32 that covers an opening at the top of the ladle 20 and an electrode 31 that can penetrate the cover 32 and be inserted into the ladle 20. Then, in the ladle refining furnace 30, desulfurization is performed to remove sulfur (S) from the molten steel, and the carbon (C) and chromium (Cr) contents are adjusted, and the temperature of the molten steel is then adjusted.

[0073] First, a desulfurization process using a ladle refining furnace 30 will be described. When the ladle 20 arrives at the ladle refining furnace 30, the cover 32 of the ladle refining furnace 30 is attached to the top of the ladle 20. Then, a desulfurization agent is introduced into the ladle 20. The desulfurization agent can be introduced into the ladle 20 by passing it through an opening in the cover 32. The desulfurization agent is preferably at least one of a first material containing quicklime (CaO) and alumina (Al2O3) and a second material containing fluorite (CaF2). When the desulfurization agent is introduced into the ladle 20, it reacts with sulfur (S) in the molten steel to generate a reaction byproduct containing sulfur (S). The generated reaction byproduct is absorbed by slag (SL) on the surface of the molten steel (M). This reduces the sulfur (S) content in the molten steel (desulfurization). At this time, the sulfur (S) content in the molten steel is set to 0.015 wt% or less, which can be adjusted by controlling the amount of desulfurization agent added.

[0074] Once desulfurization is complete, the molten steel M contained in the ladle 20 is sampled and its carbon (C) and chromium (Cr) contents are measured. The carbon (C) and chromium (Cr) contents in the molten steel M are then adjusted according to the carbon (C) and chromium (Cr) contents. For example, if the carbon (C) content is low, less than 0.75 wt%, a recarburizer containing carbon (C) is introduced into the ladle 20. The amount of the recarburizer introduced is adjusted so that the carbon (C) content in the molten steel M is 0.75 wt% or more. Conversely, if the measured carbon (C) content is greater than 0.95 wt%, solid oxygen, e.g., iron ore such as FeO, is introduced into the ladle 20. The amount of solid oxygen introduced is adjusted so that the carbon (C) content in the molten steel M is 0.95 wt% or less.

[0075] As another example, if the measured chromium (Cr) content is low, less than 4.5 wt%, a ferroalloy containing chromium (Cr) is introduced into the ladle 20. Conversely, if the measured chromium (Cr) content is higher than 5.5 wt%, solid oxygen, e.g., iron ore such as FeO, is introduced into the ladle 20. At this time, the amount of solid oxygen introduced is adjusted so that the chromium (Cr) content in the molten steel is 5.5 wt% or less.

[0076] In the above, solid oxygen is introduced into the ladle 20 connected to the ladle refining furnace 30 when the carbon (C) content exceeds 0.95 wt% or the chromium (Cr) content exceeds 5.5 wt%. However, the present invention is not limited to this, and oxygen may be blown into the molten steel M in the evacuation device 40 used in the subsequent process to adjust the carbon (C) content to 0.95 wt% or less, or the chromium (Cr) content to 5.5 wt% or less.

[0077] After the molten steel M is tapped into the ladle 20, the ladle 20 is moved to the next process. However, the temperature of the molten steel M gradually decreases while the ladle 20 is moving. That is, even if the temperature of the molten steel M is adjusted to 1650°C to 1750°C before the molten steel in the converter 11 is tapped into the ladle 20, the temperature of the molten steel M decreases as the ladle 20 moves to the next process.

[0078] Therefore, it is necessary to heat the molten steel M in the ladle 20 before transferring the ladle 20 to the evacuation device 40 used in the subsequent process. To this end, power is supplied to the electrodes 31 to generate an arc and heat from the electrodes 31, and the molten steel is heated by the generated arc and heat. At this time, the molten steel M is heated to a starting target temperature, which may be 1560°C to 1600°C. Here, the starting target temperature refers to the target temperature of the molten steel M contained in the ladle 20 when the ladle 20 is removed from the ladle refining furnace 30 and departs for the subsequent process. The temperature of the molten steel M can be adjusted to the starting target temperature by controlling at least one of the magnitude of the power supplied to the electrodes 31 and the heating time of the molten steel using the electrodes 31.

[0079] On the other hand, when the ladle 20 removed from the ladle refining furnace 30 is destined for the subsequent process, if the temperature of the molten steel M (hereinafter referred to as the starting temperature) is less than 1560°C or exceeds 1600°C, the temperature of the molten steel M may deviate from the target casting temperature when the ladle 20 reaches the tundish of the casting machine. Here, the target casting temperature refers to the target temperature of the molten steel received in the tundish, and the target casting temperature may be 1520°C to 1550°C. If the starting temperature is less than 1560°C, the temperature of the molten steel M may be less than 1520°C when the ladle reaches the tundish. Also, if the starting temperature is less than 1600°C, the temperature of the molten steel may exceed 1550°C when the ladle reaches the tundish.

[0080] If the temperature of the molten steel M is below 1520°C when the ladle 20 reaches the tundish, there is a risk of clogging the nozzle that supplies the molten steel M from the tundish to the mold. Furthermore, if the temperature of the molten steel exceeds 1600°C when the ladle 20 reaches the tundish, there is a risk of breakout, in which the solidified shell bursts and the molten steel spills out as the molten steel solidifies in the casting machine. Therefore, the starting temperature of the molten steel M in the ladle refining furnace 30 is adjusted to 1560°C to 1600°C, so that the temperature of the molten steel is 1520°C to 1550°C when the ladle 20 reaches the tundish.

[0081] The reason why the temperature adjusting step (S130) is performed using a ladle refining furnace (LF) used in a carbon steelmaking process is that it can effectively increase the temperature of the molten steel (M) without contaminating it. That is, in a stainless steelmaking process, when increasing the temperature of the molten steel, a metal such as aluminum (Al) is added to the molten steel to cause an exothermic reaction and increase the temperature. That is, in a stainless steelmaking process, a separate heating device is not used to increase the temperature of the molten steel. For this reason, in this embodiment, the temperature of the molten steel (M) is adjusted using a ladle refining furnace (LF) used to increase the temperature of the molten steel in a carbon steelmaking process. Therefore, the temperature of the molten steel (M) can be increased without adding any other material to the molten steel (M). That is, the temperature of the molten steel (M) can be increased without changing the composition of the molten steel (M).

[0082] Once the adjustment of the components and temperature in the ladle refining furnace 30 is completed, the ladle 20 is removed from the ladle refining furnace 30. Next, the ladle 20 removed from the ladle refining furnace 30 is moved to the evacuation device 40. Then, using the evacuation device 40, degassing is performed to remove nitrogen (N2) and hydrogen (H2) contained in the molten steel M (S140).

[0083] The evacuation device 40 is a reflux vacuum degassing (RH) device 40 used in the process of making carbon steel. Referring to FIG. 2(g), the RH device 40 may include a vessel 41 having an internal space, a pump (not shown) connected to the vessel 41 so as to adjust the pressure inside the vessel 41, a pair of reflux pipes 42a, 42b arranged in parallel on the left and right so as to be able to fit inside the ladle 20 and connected to the lower part of the vessel 41, and a lance 43 fitted to the upper part of the vessel 41 to blow oxygen. One of the pair of reflux pipes 42a, 42b, 42a, is an upflow pipe through which molten steel in the ladle 20 rises, and the other reflux pipe 42b is a downflow pipe through which the molten steel rising from the upflow pipe descends toward the ladle 20.

[0084] A process for removing gas from molten steel M using the RH device 40 will now be described. First, the ladle 20 is positioned below the pair of reflux pipes 42a, 42b, which are then connected to each other and sealed. The interior of the vessel 41 is then reduced in pressure to a vacuum, e.g., 0.2 torr or less, and the ladle 20 is then raised to immerse the pair of reflux pipes 42a, 42b in the molten steel in the ladle 20. An inert gas, e.g., argon (Ar), is then blown into the pair of reflux pipes 42a, 42b to circulate the molten steel M within the vessel 41. The circulating molten steel is exposed to the vacuum atmosphere within the vessel 41, which causes nitrogen (N2) and hydrogen (H2) gases in the molten steel to be exhausted to the outside of the vessel 41. For this reason, the nitrogen (N) and hydrogen (H2) contents in the molten steel M are reduced, and at this time, the nitrogen (N2) content is set to 0.015 wt% or less and the hydrogen (H2) content is set to 0.0005 wt% or less.

[0085] The reason why the hydrogen (H2) content is adjusted to 0.0005 wt% or less in the degassing step (S140) will be explained below.

[0086] When manufacturing a roll, it is necessary to adjust the hydrogen (H2) content of the roll to 0.0002 wt% or less. This is because hydrogen (H2) contained inside the roll tends to accumulate in inclusions and segregations contained in the roll, which may cause internal cracks in the roll when the roll is actually used. Here, actually using the roll may refer to using the manufactured roll to roll an object to be rolled, such as a cast slab. For this reason, the hydrogen (H2) content of the roll is adjusted to 0.0002 wt% or less. When the hydrogen (H2) content of the roll is 0.0002 wt% or less, no or very little cracks occur inside the roll due to hydrogen (H2). Furthermore, since it is practically difficult to completely remove hydrogen from the roll, the hydrogen content is adjusted to 0.0002 wt% or less.

[0087] In order to achieve a hydrogen (H2) content of 0.0002 wt% or less in the roll, it is preferable to reduce the hydrogen (H2) content in the molten steel to 0.0005 wt% or less in the degassing step (S140). In other words, if the hydrogen (H2) content in the molten steel M cannot be adjusted to 0.0005 wt% or less in the degassing step (S140), it is difficult to adjust the hydrogen (H2) content to 0.0002 wt% or less in the subsequent step of heating the roll to remove hydrogen (FIG. 4(c)). In other words, it is difficult to manufacture a roll with a hydrogen (H2) content adjusted to 0.0002 wt% or less. Therefore, the hydrogen (H2) content in the molten steel M is adjusted to 0.0005 wt% or less in the degassing step (S140).

[0088] Furthermore, in the degassing process (S140), the nitrogen (N2) content in the molten steel M is adjusted to 0.015 wt% or less. If the nitrogen (N2) content in the molten steel M exceeds 0.015 wt%, precipitates due to nitrogen (N2) will be generated when the slab is produced, which may cause a large amount of cracking in the slab. Therefore, the nitrogen (N2) content in the molten steel M is adjusted to 0.015 wt% or less using the degassing process (S130).

[0089] In this way, the hydrogen (H2) content in the molten steel M can be adjusted to 0.0005 wt% or less and the nitrogen (N2) content to 0.015 wt% or less by controlling the pressure and treatment time of the vessel 41 of the RH device 40. That is, by adjusting the pressure of the vessel 41 to 0.2 torr or less and treating the molten steel while maintaining this pressure for 20 minutes or more, it is possible to produce molten steel having a hydrogen (H2) content of 0.0005 wt% or less and a nitrogen (N2) content of 0.015 wt% or less.

[0090] The reason why the RH device 40 is used as the vacuum evacuation device 40 when performing degassing is that other types of vacuum evacuation devices have relatively low pressure adjustment capabilities. That is, the RH device 40 used in the carbon steelmaking process can adjust the pressure inside the vessel 41 to 2 torr or less, while the vacuum oxygen decarburization (VOD) device, which is a vacuum evacuation device used in the stainless steelmaking process, can only reduce the pressure to a maximum of 3 to 4 torr. Furthermore, the lower the pressure, i.e., the higher the vacuum, the more advantageous it is for removing hydrogen (H2) and nitrogen (N2) from molten steel. For this reason, the RH device 40 used for degassing in the carbon steelmaking process is used. Therefore, hydrogen (H2) and nitrogen (N2) can be easily removed from the molten steel M, and hydrogen (H2) and nitrogen (N2) can be removed more effectively than when using other vacuum evacuation devices, such as a vacuum oxygen decarburization (VOD) device.

[0091] As described above, in the converter treatment step (S110), a converter 11 used in the stainless steelmaking process is used. That is, when preparing molten steel for manufacturing a rolling roll having a high chromium (Cr) content, a converter 11 used in the stainless steelmaking process is used. Therefore, molten steel for manufacturing a rolling roll having a high chromium (Cr) content can be prepared without contaminating converters for other steel types. Furthermore, when a converter for manufacturing stainless steel that was used to prepare molten steel for manufacturing a rolling roll is used to prepare molten steel for stainless steel again, chromium remaining in the converter for manufacturing stainless steel does not act as an impurity.

[0092] A ladle refining furnace 30 used in the steelmaking process of carbon steel is used as a device for adjusting the composition and raising the temperature of the molten steel M tapped from the converter 11. That is, the temperature of the molten steel M can be raised without adding other materials to the molten steel. Therefore, the temperature of the molten steel can be raised without changing the composition of the molten steel.

[0093] Furthermore, when degassing is performed using the vacuum device 40, the RH device 40 used in the carbon steelmaking process is used. That is, degassing is performed using the RH device 40, which can adjust the vessel pressure to a low level of 0.2 torr or less. Therefore, nitrogen (N2) and hydrogen (H2) can be effectively removed from the molten steel.

[0094] Furthermore, when preparing molten steel M for manufacturing rolls, a converter 11 for making stainless steel, a ladle refining furnace 30 for making carbon steel, and an RH device 40 are used, without providing a new converter, heating device, or vacuuming device. This eliminates the need to provide separate devices for preparing molten steel M for manufacturing rolls, resulting in an effect of reducing the costs involved.

[0095] 3(a) to 3(d) are diagrams showing the operation of the casting device of the present invention in order.

[0096] First, the casting device 100 will be described with reference to (a) to (d) of FIG.

[0097] 3(a) to 3(d), the casting apparatus 100 is a vertical casting apparatus 100 that withdraws the slab S vertically from the ground when withdrawing it from the mold 120. The casting apparatus according to the embodiment may be an apparatus capable of producing a thick slab (i.e., a thick plate) having a thickness of, for example, about 700 mm.

[0098] As shown in (a) to (d) of Figure 3, the casting device 100 comprises a mold 120 capable of solidifying molten steel M supplied therein, a support unit 171 that can be inserted into the mold 120 and can be raised and lowered, a drive unit 172 connected to the support unit 171 so as to provide lifting power, a first magnetic field generating unit 150a located outside the lateral direction of the mold 120, a second magnetic field generating unit 150b located below the mold 120 and outside the lateral direction of the mold 120, and a heating unit 160 located outside the lateral direction of the mold 120 between the mold 120 and the second magnetic field generating unit 150b.

[0099] The casting device 100 also includes a cooling section 140 that extends in a direction perpendicular to the ground below the mold 120 and is capable of solidifying the slab S pulled out below the mold 120 by spraying cooling water onto it, a rotating section 190 that is arranged below the second magnetic field generating section 150b so as to receive and rotate the slab S supported by the support section 171, and a moving section 180 that is arranged below the second magnetic field generating section 150b facing the rotating section 190 so as to push and move the slab S supported by the support section 171 toward the rotating section 190.

[0100] The casting apparatus 100 may also include a tundish 110 located above the mold 120 to supply molten steel M to the mold 120, and a nozzle 130 connected to the bottom of the tundish 110 so as to supply molten steel M to the mold 120.

[0101] For ease of explanation, in Fig. 3, the rotating part 190 is shown only in Fig. 3(d) and is omitted in Fig. 3(a) to (c). However, even in the cases such as Fig. 3(a) to (c), the rotating part 190 is disposed below the cooling part so as to face the moving part 180.

[0102] The mold 120 is a means for receiving the liquid molten steel M from the tundish 110 and primarily solidifying it into a predetermined shape. The mold 120 may be provided with a cooling water pipe (not shown) therein through which cooling water is circulated. When the molten steel is supplied to the mold 120 and primarily solidified, the molten steel M is in a semi-solidified state in which a solidified region A where the molten steel M is solidified and a non-solidified region B coexist.

[0103] Before molten steel M is supplied to the mold 120, the support part 171 is inserted into the mold 120 so as to close the lower opening of the mold 120. When molten steel M is supplied into the mold 120 with the support part 171 closing the lower opening of the mold 120, the molten steel M begins to solidify on the support part 171. Therefore, a semi-solidified slab S is supported on the support part 171. When the drive part 172 is operated to lower the support part 171, the support part 171 lowers to the lower side of the mold 120 while supporting the semi-solidified slab S. In other words, the lowering of the support part 171 causes the slab S inside the mold 120 to be pulled out to the lower side of the mold 120. Such a support part may be plate-shaped.

[0104] The driving unit 172 is a means for raising and lowering the support unit 171, and can be connected to the lower part of the support unit 171. The driving unit 172 may include a power source 172-1 that provides a driving force for raising and lowering, and a driving member 172-2 that connects the power source 172-1 and the support unit 171 so that the support unit 171 can be raised and lowered by the driving force transmitted from the power source 172-1.

[0105] The power source 172-1 may be a device equipped with a hydraulic piston. Needless to say, the power source 172-1 is not limited to the above example, and any means may be used as long as it is capable of raising and lowering the driving member 172-2.

[0106] One end of the driving member 172-2 is connected to the power source 172-1, and the other end is connected to the support portion 171. Such a driving member 172-2 may have a shape that extends in the vertical direction, for example. Alternatively, the driving member 172-2 may be provided so that the height of one end connected to the power source 172-1 is fixed, and the height of the other end connected to the support portion 171 can be raised or lowered by the operation of the power source 172-1.

[0107] As described above, the driver 172 is a means for raising and lowering the support 171 and can adjust the speed at which the support 171 is raised and lowered. In particular, by adjusting the speed at which the support 171 is lowered, the speed at which the slab S supported by the support 171 is withdrawn below the mold 120 can be adjusted. In this case, the speed at which the slab S is withdrawn below the mold 120 may refer to the casting speed. When the driver 172 adjusts the lowering speed of the support 171, it adjusts its operation so that the support 171 descends at a speed of 0.04 m / min or less. More specifically, it adjusts its operation so that the support 171 descends at a speed of 0.01 m / min to 0.04 m / min. In other words, the driver 172 adjusts the lowering speed of the support 171 so that the casting speed is 0.04 m / min or less, more specifically, 0.01 m / min to 0.04 m / min.

[0108] In this way, the casting speed is adjusted to 0.04 m / min or less in order to suppress or prevent the occurrence of segregation in the slab S and suppress or prevent a decrease in the production rate. That is, if the casting speed exceeds 0.04 m / min, the unsolidified molten steel M inside the slab S may not be able to solidify sufficiently, which may result in segregation. Furthermore, if the casting speed is less than 0.01 m / min, there is a problem that the production rate of the slab S may decrease. Therefore, it is preferable to adjust the casting speed to 0.01 m / min to 0.04 m / min.

[0109] The cooling section 140 includes a plurality of rolls 141 arranged below the mold 120 in the direction in which the support section 171 rises and falls, and nozzles (not shown) located between the plurality of rolls 141 to spray cooling water onto the cast piece drawn out below the mold.

[0110] The rolls 141 can be arranged in a direction perpendicular to the ground. Each of the rolls 141 is arranged so that it can rotate due to the downward force of the slab S when the slab S is lowered downward by the support parts 171. Therefore, the slab S drawn out to the bottom of the mold 120 is lowered in a direction perpendicular to the ground by the lowering of the support parts 171 and the rolls 141.

[0111] Nozzles for spraying cooling water are arranged between the multiple rolls 141. Therefore, the slab S drawn out to the lower side of the mold 120 is secondarily cooled by the cooling water sprayed from the nozzles while being lowered by the support parts 171.

[0112] When cooling water is sprayed onto the slab S drawn below the mold 120 to cause secondary solidification of the slab S, the spray flow rate of the cooling water is adjusted so that the surface temperature of the slab S reaches 800°C to 900°C. That is, the spray flow rate of the cooling water is adjusted so that the entire surface of the slab S below the mold 120 reaches 800°C to 900°C. More specifically, the spray flow rate of the cooling water is adjusted so that the surface temperature of the slab S from the top to the bottom is uniform at 800°C to 900°C. For this purpose, among the multiple nozzles arranged vertically, the spray amount of cooling water can be adjusted so that the lower the nozzle, the smaller the spray amount of cooling water.

[0113] The surface temperature of the slab S is adjusted to 800°C to 900°C in order to suppress or prevent the occurrence of surface cracks on the slab S and to prevent the occurrence of bulging. That is, if the surface temperature of the slab S is below 800°C, cracks may occur on the surface of the slab S due to overcooling. Conversely, if the surface temperature of the slab S exceeds 900°C, the strength of the solidified shell, which is the surface portion of the slab S, is low, and bulging may occur, in which the solidified shell bulges out. Therefore, the surface temperature of the slab S pulled out of the mold 120 is adjusted to 800°C to 900°C.

[0114] If the temperature of the molten steel M supplied to the mold 120 is low, the molten steel M solidifies and the molten steel becomes congested, which causes segregation in the slab S. For this reason, in order to prevent the temperature of the molten steel from dropping, mold flux, which is a heat-retaining agent, is applied to the surface of the molten steel supplied to the mold 120. However, mold flux alone cannot sufficiently prevent the temperature of the molten steel from dropping.

[0115] Therefore, a first magnetic field generator 150a is provided outside the mold 120 to flow the molten steel M inside the mold 120 and prevent a drop in the temperature of the molten steel M. The first magnetic field generator 150a is located outside the lateral direction of the mold 120 and generates a magnetic field. Here, the outside of the lateral direction of the mold 120 may refer to the outside of the outer surface of the mold 120, which is the surface opposite the inner surface of the mold 120 that comes into contact with the molten steel M. In other words, the first magnetic field generator 150a is disposed outside the mold 120 so as to face the outer surface of the mold 120. In other words, the first magnetic field generator 150a can be disposed outside the lateral direction of the mold 120 so as to surround the mold 120. For this purpose, the first magnetic field generator 150a may be provided in a hollow shape extending along the outer surface of the mold 120. The first magnetic field generating unit 150a may include a coil that is disposed in the trunk and inside the trunk and generates a magnetic field when power is supplied.

[0116] When a magnetic field is generated by the first magnetic field generating unit 150a, the generated magnetic field causes the molten steel M inside the mold 120 to flow. Therefore, the molten steel M inside the mold 120 flows due to the magnetic field, and a drop in temperature due to the flow of the molten steel M can be suppressed or prevented. When the molten steel M flows, the components contained in the molten steel M are mixed uniformly or homogeneously. This can suppress or prevent the occurrence of segregation, which is formed when specific components accumulate or gather in a predetermined region inside the cast slab.

[0117] The magnetic field generated by the first magnetic field generating unit 150a varies depending on the magnitude of the current corresponding to the power supplied to or voltage applied to the first magnetic field generating unit 150a. Therefore, the power supplied to or voltage applied to the first magnetic field generating unit 150a is adjusted so that a current of a target magnitude flows through the coil of the first magnetic field generating unit 150a. In this manner, a magnetic field can be generated in the first magnetic field generating unit 150a by causing a current to flow through the first magnetic field generating unit 150a. The magnetic field generated by the first magnetic field generating unit 150a can cause the molten steel M inside the mold 120 to flow, thereby suppressing or preventing a drop in the temperature of the molten steel M and suppressing or preventing mold flux on the surface of the molten steel M from being mixed into the molten steel.

[0118] On the other hand, if the current flowing through the first magnetic field generating unit 150a is too low, the molten steel M inside the mold 120 may not flow, which may cause the temperature of the molten steel M to drop and segregate the slab S. Conversely, if the current flowing through the first magnetic field generating unit 150a is too high, the flow rate of the molten steel inside the mold 120 may be too high, causing mold flux to be mixed into the molten steel. Mold flux mixed into the molten steel M is an impurity and may cause cracks in the slab. Therefore, the magnitude of the current flowing through the first magnetic field generating unit 150a is adjusted by adjusting the magnitude of the power supplied to or voltage applied to the first magnetic field generating unit 150a so as to generate a magnetic field that allows the molten steel inside the mold 120 to flow at an appropriate rate.

[0119] When the molten steel M solidifies inside the mold 120, the molten steel M supplied inside the mold 120 does not solidify all at once. That is, the molten steel M solidifies sequentially from the peripheral portions in the width and length directions of the mold 120 toward the central region. Therefore, inside the mold 120, the molten steel solidifies in the peripheral portions in the width and length directions and becomes a solidified shell or a solid state, while the central region in the width and length directions of the mold 120 is in an unsolidified state where the molten steel M exists in a liquid state. In other words, the cast piece produced by solidifying a portion of the molten steel inside the mold 120 is in a semi-solidified state where a solidified region A and a unsolidified region B exist.

[0120] Furthermore, when the semi-solidified slab S is withdrawn below the mold 120, the slab S is secondarily cooled by cooling water sprayed from nozzles in the cooling unit 140. At this time, since the cooling unit 140 is located laterally of the slab S, the slab S is solidified sequentially from the peripheral portions in the width and length directions toward the central region. Therefore, the slab S withdrawn below the mold 120 may be in a semi-solidified state in which a solidified region A and a non-solidified region B exist. Then, as time passes, the molten steel M in the non-solidified region B solidifies, resulting in the production of a completely solidified slab without the non-solidified region B.

[0121] In the slab S withdrawn to the bottom of the mold 120, the accumulated solidification time becomes shorter as it moves toward the top closer to the mold 120, and becomes longer as it moves toward the bottom. Therefore, in the slab S withdrawn to the bottom of the mold 120, the area of ​​the non-solidified region B becomes wider as it moves toward the top closer to the mold 120, and the area of ​​the non-solidified region B becomes narrower as it moves toward the bottom. In other words, in the slab S withdrawn to the bottom of the mold 120, the area of ​​the solidified region A becomes narrower as it moves toward the top closer to the mold 120, and the area of ​​the solidified region A becomes wider as it moves toward the bottom.

[0122] As described above, a non-solidified region B where the molten steel M does not solidify exists above the slab S withdrawn below the mold 120. That is, the slab S contains unsolidified molten steel M. When the slab S is withdrawn below the mold 120, the slab S is solidified by the cooling water sprayed from the cooling section 140. However, if the unsolidified molten steel M present in the slab S solidifies without being congested, segregation may occur within the slab S.

[0123] Therefore, a second magnetic field generator 150b is provided below the mold 120 so as to allow the unsolidified molten steel M inside the slab S withdrawn below the mold 120 to flow. The second magnetic field generator 150b is located below the mold 120, outside the lateral direction of the mold 120, and generates a magnetic field. More specifically, the second magnetic field generator 150b may be arranged below the mold 120 so as to face the first magnetic field generator 150a in the vertical direction. Even more specifically, the second magnetic field generator 150b may be arranged below the mold 120 so as to be located outside the lateral direction of the cooling unit 140. For this purpose, the second magnetic field generator 150b may be provided in a hollow shape. Furthermore, the second magnetic field generator 150b may be provided to have the same configuration as the first magnetic field generator 150a. That is, the second magnetic field generating unit 150b may be provided with a coil that is disposed in the trunk and inside the trunk and generates a magnetic field when power is supplied.

[0124] When a magnetic field is generated by the second magnetic field generating unit 150b, the generated magnetic field is applied to the slab S. As a result, the liquid molten steel inside the slab S flows due to the magnetic field. As a result, the molten steel M inside the slab S can solidify while flowing, which can suppress or prevent segregation from occurring inside the slab S.

[0125] The magnetic field generated by the second magnetic field generating unit 150b varies depending on the magnitude of the current corresponding to the power supplied to or voltage applied to the second magnetic field generating unit 150b. Therefore, the power supplied to or voltage applied to the second magnetic field generating unit 150b is adjusted so that a current of a target magnitude flows through the coil of the second magnetic field generating unit 150b. In this way, by causing a current to flow through the second magnetic field generating unit 150b, a magnetic field can be generated in the second magnetic field generating unit 150b. The magnetic field generated by the second magnetic field generating unit 150b can cause the molten steel inside the slab S to flow, thereby suppressing or preventing the mold flux from being mixed into the molten steel M.

[0126] On the other hand, if the current flowing through the second magnetic field generating unit 150b is too low, the molten steel M inside the slab S may not flow or may not flow sufficiently, which may result in segregation inside the slab S. Conversely, if the current flowing through the first magnetic field generating unit 150a is too high, the flow rate of the molten steel M may be too high, causing mold flux to be mixed into the molten steel. Mold flux mixed into the molten steel M is an impurity and may cause cracks in the slab S. Therefore, when generating a magnetic field using the second magnetic field generating unit 150b, the current flowing through the second magnetic field generating unit 150b is adjusted by adjusting the magnitude of the power supplied to or the voltage applied to the second magnetic field generating unit 150b so as to generate a magnetic field that allows the molten steel M inside the slab S to flow at an appropriate rate.

[0127] If the time for which the second magnetic field generating unit 150b is operated to generate a magnetic field is too short, the slab S may not be sufficiently solidified. That is, casting may end with unsolidified molten steel remaining inside the slab S. Conversely, if the time for which the second magnetic field generating unit 150b is operated to generate a magnetic field is too long, the magnetic field may be applied to areas where there is no unsolidified molten steel, i.e., the lower part of the slab. That is, the magnetic field may be applied to areas where application of a magnetic field is not necessary. Therefore, when generating a magnetic field using the second magnetic field generating unit 150b, the time for applying the magnetic field to the slab is adjusted so that the unsolidified molten steel inside the slab S can be sufficiently solidified, while preventing the magnetic field from being unnecessarily applied to the solidified areas.

[0128] As explained above, the slab withdrawn below the mold 120 has a larger non-solidified region toward the top. Here, the bottom end of the slab S is the region that is first withdrawn from the mold 120, and the top end of the slab S is the region that is last withdrawn from the mold 120. For this reason, the top end of the slab S withdrawn below the mold 120 is referred to as the end portion of the slab S.

[0129] When the slab S drawn to the bottom of the mold 120 solidifies, the unsolidified molten steel at the end of the slab solidifies before the molten steel M below the end of the slab. This solidification of the molten steel M at the end of the slab occurs before the molten steel M below the end of the slab. This leads to shrinkage cavities at the end of the slab due to solidification shrinkage. Since the end of the slab containing shrinkage cavities cannot be used as a product, the end of the slab containing shrinkage cavities is cut after casting is completed. This reduces the actual yield of the slab by an amount corresponding to the length of the cut end. Furthermore, because the cut end is discarded, material is consumed in an amount corresponding to the cut length with each operation, resulting in cost waste.

[0130] Therefore, a heating unit 160 capable of heating the end portion of the slab is provided below the mold 120 so as to suppress or prevent solidification shrinkage of the end portion of the slab. The heating unit 160 is disposed between the mold 120 and the second magnetic field generating unit 150b and outside the lateral direction of the mold 120. More specifically, the heating unit 160 may be disposed below the mold 120 so as to face the first magnetic field generating unit 150a and the second magnetic field generating unit 150b in the vertical direction. The heating unit 160 may also be disposed so as to be located outside the lateral direction of the cooling unit 140. For this purpose, the heating unit 160 may be hollow. Such a heating unit 160 may include a body and a heating element disposed inside the body that generates heat by supplied power.

[0131] When the heating unit 160 operates and generates heat, the heat heats or inductively heats the molten steel M inside the end portion of the slab. This can delay the solidification of the molten steel M at the end portion of the slab. That is, the molten steel at the end portion of the slab can be adjusted to solidify more slowly than the molten steel below it. This can suppress or prevent shrinkage cavity defects from occurring at the end portion of the slab.

[0132] The heat generated in the heating section 160 varies depending on the power or voltage supplied to the heating section 160. Therefore, the power supplied to the heating section 160 is adjusted so that heat of a target temperature is generated in the heating section 160. In this way, the end portion of the slab can be heated by supplying power to the heating section 160 to generate heat. This makes it possible to suppress or prevent solidification shrinkage of the end portion of the slab.

[0133] On the other hand, if the power supplied to the heating unit 160 is too low, the temperature of the heat generated in the heating unit 160 is low and the molten steel M at the end of the slab cannot be sufficiently heated, which may cause solidification shrinkage at the end of the slab and lead to shrinkage cavities. Conversely, if the power supplied to the heating unit 160 is too high, the temperature of the heat generated in the heating unit 160 is too high, which may cause remelting of the lower part of the slab in addition to the end of the slab. Therefore, when heating the end of the slab using the heating unit 160, the power supplied to the heating unit 160 is adjusted so that the end of the slab can be sufficiently heated without causing the lower part of the slab to be heated and remelted.

[0134] Furthermore, if the time for heating the end portion of the slab using the heating section 160 is too short, the end portion of the slab may not be heated sufficiently. As a result, the molten steel M at the end portion of the slab may solidify earlier than the molten steel M below the end portion of the slab, causing solidification shrinkage at the end portion of the slab, which may result in shrinkage cavities. Conversely, if the time for heating the end portion of the slab using the heating section 160 is too long, the molten steel at the end portion of the slab has already solidified, making it unnecessary to heat the end portion of the slab using the heating section. Therefore, when heating the end portion of the slab using the heating section 160, the heating time for the end portion of the slab is adjusted so that the end portion of the slab can be sufficiently heated without heating the end portion of the slab that has already solidified.

[0135] As described above, in the embodiment, when the molten steel is solidified inside the mold 120, the first magnetic field generating unit 150a is used to flow the molten steel inside the mold 120. This makes it possible to suppress or prevent a drop in the temperature of the molten steel M inside the mold 120, thereby suppressing or preventing the occurrence of segregation in the slab S.

[0136] Furthermore, when the slab drawn below the mold 120 solidifies, the unsolidified molten steel inside the slab S is made to flow using the second magnetic field generating unit 150b disposed below the mold 120. Therefore, when the slab solidifies outside the mold 120, the unsolidified molten steel inside the slab S can flow and solidify without congestion. This can suppress or prevent segregation in the slab S due to congestion of the unsolidified molten steel.

[0137] Meanwhile, in the past, electroslag remelting (ESR) was used to prevent segregation in ingots used to manufacture mill rolls. The electroslag remelting (ESR) process will be briefly described below. First, molten steel with a smooth top is solidified to produce an electrode rod for use in manufacturing mill rolls. An arc is then generated using the produced electrode rod to remelt the electrode rod, and the remelted molten steel is then dropped downward like a water droplet and solidified to produce an ingot. While this process can suppress or prevent segregation in the ingot, it requires complex processes, such as converting the ingot produced as described above into an electrode rod, remelting the electrode rod, and resolidifying the remelted molten steel. This process, therefore, requires significant time and cost.

[0138] In contrast, the first and second magnetic field generators M provided in the casting apparatus 100 can be used to fluidize the molten steel in the mold 120 and the unsolidified molten steel M in the slab S, thereby suppressing or preventing segregation within the slab S. Therefore, compared to producing an ingot using a conventional electroslag remelting (ESR) method, producing a slab using the method according to the present embodiment is simpler, takes less time, and is less costly. Furthermore, the amount of power required to operate the first and second magnetic field generators is smaller than the amount of power required to generate an arc between electrodes in the conventional electroslag remelting (ESR) method. Therefore, compared to producing an ingot using a conventional electroslag remelting (ESR) method, the amount of electrical energy consumed when producing a slab can be reduced.

[0139] Furthermore, by using the end portion of the slab to heat the heating section 160, the solidification of the molten steel M at the end portion of the slab can be delayed. This makes it possible to suppress or prevent solidification shrinkage at the end portion of the slab, thereby making it possible to suppress the occurrence of shrinkage cavity defects at the end portion of the slab.

[0140] Returning to FIG. 3 again, the moving unit 180 and the rotating unit 190 will be described.

[0141] The moving unit 180 is a means for pushing the strand S that has been withdrawn below the mold 120 from one side and moving it toward the rotating unit 190. That is, the moving unit 180 is withdrawn below the mold 120 and supported vertically on the support unit 171, and pushes the solidified strand S to deliver it to the rotating unit 190. Such a moving unit 180 may be, for example, a means that is driven horizontally relative to the ground, and may include, for example, a hydraulic or pneumatic cylinder.

[0142] The rotator 190 receives the slab S that has been separated from the support unit 171 by the mover 180, and rotates the slab S. That is, the rotator 190 receives the slab that is supported on the support unit 171 in a direction perpendicular to the ground, and rotates the slab parallel to the ground.

[0143] The rotating unit 190 is disposed horizontally opposite the moving unit 180. The rotating unit 190 is capable of receiving and supporting the slab S separated from the support unit 171, and includes a rotatable rotating table 191 and a rotating member 192 connected to the rotating table 191 so that the rotating table 191 can rotate.

[0144] The rotary table 191 includes a first table 191-1 extending in one direction and a second table 191-2 extending in a direction intersecting the extending direction of the first table 191-1 and having one end connected to the first table 191-1.

[0145] With respect to the slab S supported vertically on the support portion 171 as a reference, the first table 191-1 is a means for supporting the side surface of the slab S, and the second table 191-2 is a means for supporting the lower surface of the slab S. The first table 191-1 may be provided so as to have a longer extension length than the second table 191-2. The second table 191-2 is preferably provided so as to have the same area as the lower surface of the slab S or a larger area. A plurality of rotatable rollers 191-3 may be provided on the first table 191-1, and the plurality of rollers 191-3 are aligned in the direction in which the first table 191-1 extends.

[0146] One end of the first table 191-1 and one end of the second table 191-2 can be connected to each other by a rotating member 192. The rotating table 191 can be rotated or tilted by the rotating member 192 as shown in FIG. 3(d). That is, when the slab S is supported vertically on the support part 171, the first table 191-1 of the rotating part is perpendicular to the ground, and the second table 191-2 is arranged parallel to the ground. When the slab S supported on the support part 171 is transferred to the rotating part 190, the rotating part 190 is rotated or tilted. That is, the rotating part 190 is rotated so that the first table 191-1 of the rotating part 190 is parallel to the ground, and the second table 191-2 is perpendicular to the ground.

[0147] Although the above description has been given of a method for producing a cast piece using a vertical casting apparatus, the present invention is not limited to this, and various types of casting apparatuses including a first magnetic field generating unit 150a disposed laterally outside the mold 120, a second magnetic field generating unit 150b disposed below the mold 120, and a heating unit 160 can be used to produce the cast piece S.

[0148] 4(a) to 4(c) are process flow diagrams sequentially showing a method for manufacturing a rolling roll using a cast slab manufactured according to the present invention.

[0149] Once the slab S is produced, it is heated and softened (S300). To this end, the slab S is charged into a first heating device 200. The first heating device 200 may include a furnace having an internal space and a heater for heating the furnace. Here, the heater may include, for example, a heating element disposed inside or outside a wall constituting the furnace and capable of generating heat using supplied power. As another example, the heater may include a burner that generates heat by burning fuel.

[0150] When heating the slab S in the first heating device 200, the slab is heated to a temperature of 1100°C to 1250°C, preferably 1140°C to 1240°C. At this time, the slab is not charged into a furnace heated to 1100°C to 1250°C, but is charged into a furnace adjusted to an even lower temperature, and the temperature inside the furnace is then raised stepwise to 1140°C to 1240°C.

[0151] More specifically, the interior of the furnace is first heated to 250°C to 350°C (first temperature) and maintained at the first temperature for 3 to 5 hours, preferably 3 hours 30 minutes to 4 hours 30 minutes. The interior of the furnace is then heated to 450°C to 550°C (second temperature) and maintained at the second temperature for 5 to 7 hours, preferably 5 hours 30 minutes to 6 hours 30 minutes. The interior of the furnace is then heated to 650°C to 750°C (third temperature) and maintained at the third temperature for 3 to 5 hours, preferably 3 hours 30 minutes to 4 hours 30 minutes. The interior of the furnace is then heated to 1100°C to 1250°C (fourth temperature) and maintained at the fourth temperature for 14 to 18 hours, preferably 15 hours 30 minutes to 17 hours 30 minutes. The time required to raise the temperature inside the furnace from the first temperature to the second temperature (first temperature rise time) and the time required to raise the temperature from the second temperature to the third temperature are each preferably 3 to 4 hours, and the time required to raise the temperature inside the furnace from the third temperature to the fourth temperature is preferably 9 to 10 hours. In this way, heating the inside of the furnace to the first to fourth temperatures and maintaining the first to fourth temperatures means that the temperature of the slab charged inside the furnace is heated to the first to fourth temperatures and the temperature of the slab is maintained at the first to fourth temperatures.

[0152] The reason for raising the temperature stepwise as described above when heating the slab S from 1100°C to 1250°C is that the slab S containing a high amount of chromium, 4.5 wt% or more, has high hardness and may be prone to cracking during heating and cooling. Therefore, when heating the slab S from 1100°C to 1250°C, raising the temperature stepwise by the method described above can suppress or prevent cracking in the slab S.

[0153] After the step of heating or heat treating the slab S in the first heating device 200 is completed, the slab S is forged using the forging device 300 to manufacture a rolling roll. The forging device 300 may include, for example, an upper press section and a lower press section arranged vertically apart from each other, and a drive section connected to the upper press section and the lower press section so as to apply a predetermined force. Here, the drive section may be a cylinder driven hydraulically or pneumatically.

[0154] A method for forging a slab S using the forging apparatus 300 will be described below. First, a slab is placed between the upper and lower press sections of the forging apparatus 300. The drive unit is then operated to lower the upper press section and raise the lower press section, thereby reducing the distance between the upper and lower press sections. The drive unit is then operated to apply a further downward force to the upper press section and a further upward force to the lower press section. As time passes after the upper and lower press sections contact the slab S, the force with which the upper and lower press sections press the slab increases. As a result, the slab S placed between the upper and lower press sections is reduced, thereby reducing its thickness. When the slab thickness has decreased by an amount corresponding to the target thickness, the upper and lower press sections are separated from the slab. The target thickness may be 250 mm to 350 mm. This process is repeated multiple times.

[0155] In this way, the upper and lower pressure sections are brought into contact with the slab S and force is applied, and when the thickness of the slab S is reduced by an amount corresponding to the target thickness, the upper and lower pressure sections are separated from the slab S; this series of processes is defined as "one pass." In this embodiment, the target thickness is set to 250 mm to 350 mm. The reduction in thickness of the slab S per pass is adjusted to 250 mm to 350 mm. The above-mentioned one pass is then repeated multiple times to manufacture a rolling mill roll.

[0156] On the other hand, if the reduction in thickness of the slab S per pass is less than 250 mm, sufficient force is not applied to the central region in the thickness direction of the slab S, which may result in pores in the central region of the slab not being removed. If pores in the center in the thickness direction of the slab S are not removed, this may result in a decrease in the hardness of the rolling roll. Conversely, if the reduction in thickness of the slab S per pass exceeds 350 mm, there is a risk of ruptures occurring on the surface of the slab, which may result in defects in the rolling roll.

[0157] Once the forging is completed and the roll 141 is manufactured, the roll 141 is heated to remove hydrogen (H2). To this end, the roll 141 is placed in a second heating device 400, and the roll 141 is heated using the second heating device 400. Here, the second heating device 400 may be the same as or different from the first heating device 200 described above.

[0158] When the rolling roll 141 is heated using the second heating device 400, the rolling roll 141 is heated to 200°C to 400°C, more preferably 250°C to 350°C. The temperature of the rolling roll 141 is maintained at 200°C to 400°C for 48 hours or more, more preferably 48 hours or more and 55 hours or less.

[0159] In this way, the rolling roll 141 is heated to 200°C to 400°C, more preferably 250°C to 350°C, and while being held at this temperature for 48 hours or more, hydrogen (H2) contained in or remaining in the rolling roll 141 diffuses to become hydrogen gas, which is then discharged to the outside. As a result, the hydrogen content in the rolling roll 141 decreases. At this time, it is preferable to keep the hydrogen content in the rolling roll 141 at 0.0002 wt% or less (0 wt or more), and this can be adjusted by controlling at least one of the temperature at which the rolling roll 141 is heated and the heat treatment time.

[0160] On the other hand, hydrogen contained inside the roll 141 is likely to accumulate in inclusions and segregations contained in the roll 141, which may cause internal cracks in the roll when the roll 141 is actually used. Here, actually using the roll may mean using the manufactured roll 141 to roll an object to be rolled, for example, a cast S.

[0161] Therefore, the manufactured roll 141 is heated at 200°C to 400°C for 48 hours or more to remove hydrogen so that the hydrogen content in the roll 141 is 0.0002 wt% or less. At this time, if the hydrogen content in the roll 141 is 0.0002 wt% or less, cracks due to hydrogen do not occur inside the roll 141. Since it is practically difficult to completely remove hydrogen from the roll 141, the hydrogen content is adjusted to 0.0002 wt% or less.

[0162] Hereinafter, a method for manufacturing a rolling roll according to an embodiment of the present invention will be collectively described with reference to Figures 1 to 4. In this case, the content that overlaps with the content described above will be omitted or will be described briefly.

[0163] First, molten steel M for manufacturing mill rolls is prepared (S100). For this purpose, the molten steel M is first charged into a converter 11 for manufacturing stainless steel. Then, as shown in FIG. 2(a), chromium (Cr) alloy ferroalloy is charged into the converter 11. At this time, the amount of chromium (Cr) alloy ferroalloy charged is adjusted so that the chromium (Cr) content in the molten steel inside the converter 11 is 4.5 wt% to 5.5 wt%.

[0164] Next, as shown in Fig. 2(b), oxygen is blown into the converter 11 using a lance 12. That is, oxygen is blown into the molten steel M in the converter 11 to remove carbon (C) and phosphorus (P). At this time, the carbon (C) content in the molten steel M is set to 0.75 wt% to 0.95 wt%, and the phosphorus (P) content is set to 0.025 wt% or less.

[0165] Next, as shown in Fig. 2(c), a deoxidizer is introduced into the converter 11 to remove oxygen (O) from the molten steel M. At this time, at least one of an alloy containing silicon (Si) and an alloy containing aluminum (Al) can be used as the deoxidizer. Then, the molten steel M is deoxidized so that the content of oxygen (O) therein is 0.001 wt% or less.

[0166] Once deoxidation is complete, a reducing agent, i.e., a ferroalloy containing silicon (Si), is introduced into the converter 11, as shown in FIG. 2(d). This causes a reaction between the chromium oxide contained in the slag SL and the silicon (Si) contained in the reducing agent. As a result, the chromium oxide contained in the slag SL is reduced to chromium (Cr), and the generated chromium (Cr) is absorbed or supplied to the molten steel M. This increases the chromium (Cr) content in the molten steel M. At this time, the amount of reducing agent introduced is adjusted so that the chromium (Cr) content in the molten steel M is 4.5 wt% to 5.5 wt%.

[0167] When the reduction to chromium is completed, the molten steel M inside the converter 11 is tapped into the ladle 20 as shown in FIG. 2(e). At this time, the molten steel M is tapped into the ladle 20 for making carbon steel.

[0168] Then, the ladle 20 is moved to the temperature control device 30, i.e., the ladle refining furnace 30, and the ladle 20 is connected to the cover 32 of the ladle refining furnace 30 (FIG. 2(f)). Next, a desulfurization agent is added to the ladle 20 to remove sulfur (S) from the molten steel. At this time, the sulfur (S) content in the molten steel is adjusted to 0.015 wt% or less.

[0169] Once desulfurization is complete, the molten steel M contained in the ladle 20 is sampled and the carbon (C) and chromium (Cr) contents are measured. Then, the carbon (C) and chromium (Cr) contents in the molten steel M are adjusted according to the measured carbon (C) and chromium (Cr) contents. At this time, a recarburizer and a ferroalloy containing chromium (Cr) or solid oxygen such as iron ore are added according to the measured carbon (C) and chromium (Cr) contents, so that the measured carbon (C) content of the molten steel is 0.75 wt% to 0.95 wt% and the chromium (Cr) content is 4.45 wt% to 5.5 wt%.

[0170] Once the adjustment of the carbon (C) and chromium (Cr) components is completed, the molten steel M is heated to increase its temperature. That is, an arc and heat are generated from the electrode rod 31 to heat the molten steel contained in the ladle 20. At this time, the temperature of the molten steel is set to 1560°C to 1600°C.

[0171] When the temperature of the molten steel M reaches 1560°C to 1600°C, the ladle 20 is removed from the ladle refining furnace 30. The ladle 20 is then moved to a vacuum device, i.e., an RH device 40, as shown in FIG. 2(g). The ladle 20 is then positioned below a pair of reflux pipes 42a, 42b, which are then connected and sealed. A pump is then operated to reduce the pressure inside the vessel 41 to a vacuum, e.g., 0.2 torr or less, and the ladle 20 is then raised, immersing the pair of reflux pipes 42a, 42b in the molten steel inside the ladle 20. Argon (Ar) gas is then blown into the pair of reflux pipes 42a, 42b to circulate the molten steel inside the vessel 41. The circulating molten steel is exposed to the vacuum atmosphere inside the vessel 41, which causes nitrogen (N2) and hydrogen (H2) gases in the molten steel to be exhausted to the outside of the vessel 41. For this purpose, the nitrogen (N) and hydrogen (H2) contents in the molten steel are reduced, and at this time, the nitrogen (N2) content is set to 0.015 wt% or less and the hydrogen (H2) content is set to 0.0005 wt% or less.

[0172] After the above-described steps (a) to (g) in FIG. 2 are performed, molten steel M for manufacturing a rolling roll is prepared.

[0173] Once the molten steel M is prepared, the ladle 20 is moved to the tundish 110 of the casting apparatus 100, and the molten steel M is supplied to the tundish 110 to begin casting. To this end, first, the lower opening of the mold 120 is closed using the support 171 of the casting apparatus 100. Then, as shown in FIG. 3(a), the molten steel M from the tundish 110 is supplied to the mold 120 (S210). As a result, the molten steel M supplied to the mold 120 solidifies, and solidification begins from above the support 171 (casting process) (S210). Then, while the molten steel M from the tundish 110 is continuously supplied to the mold 120, the drive unit 172 is operated to lower the support 171. As a result, the slab S inside the mold 120 is gradually pulled downwards of the mold 120, as shown in FIG. 3(b) (S220).

[0174] As described above, while the molten steel M is continuously supplied to the mold 120, the first magnetic field generating unit 150a located outside the mold 120 is operated to generate a magnetic field. Therefore, the magnetic field generated by the first magnetic field generating unit 150a is applied to the inside of the mold 120, and the molten steel M inside the mold 120 flows due to the magnetic field. Therefore, a drop in the temperature of the molten steel M inside the mold 120 can be suppressed or prevented, and the mold flux on the surface of the molten steel M can be suppressed or prevented from being mixed into the molten steel.

[0175] While molten steel is continuously supplied into the mold 120, the driving unit 172 is operated to gradually lower the support unit 171 to the bottom of the mold 120. At this time, the speed at which the support unit 171 descends is adjusted to be 0.04 m / min or less, more specifically, adjusted to be 0.01 m / min to 0.04 m / min. That is, the casting speed is adjusted to be 0.04 m / min or less, more specifically, adjusted to be 0.01 m / min to 0.04 m / min.

[0176] As shown in FIG. 3(b), once the slab S is withdrawn below the mold 120, the withdrawn slab S undergoes secondary solidification (solidification process) by cooling water sprayed from the nozzles of the cooling unit 140 (S220). The tundish 110 continuously supplies molten steel M to the mold, and the support unit 171 continuously descends, gradually increasing the length of the slab withdrawn below the mold 120. Once the target length of the slab has been withdrawn below the mold 120, the supply of molten steel to the mold 120 is stopped. Then, as shown in FIG. 3(c), the slab is withdrawn below the mold until it is at its uppermost end, i.e., its end (S230). Once the end of the slab has been withdrawn below the mold, the second magnetic field generator 150b is activated to generate a magnetic field. Therefore, the unsolidified molten steel present inside the slab S withdrawn to the lower side of the mold 120 flows due to the magnetic field generated by the second magnetic field generating unit 150b, thereby suppressing or preventing the mold flux from being mixed into the molten steel.

[0177] In the above, it has been described that the second magnetic field generating unit 150b is operated to generate a magnetic field once the end of the slab S has been withdrawn below the mold. However, the present invention is not limited to this, and it is also possible to operate the second magnetic field generating unit 150b to generate a magnetic field once the slab begins to be withdrawn below the mold 120.

[0178] Once the end portion of the slab S is pulled out to the bottom of the mold, the heating unit 160 is operated to heat the end portion of the slab, which slows down the solidification of the unsolidified molten steel M at the end portion of the slab S. This makes it possible to suppress or prevent solidification shrinkage at the end portion of the slab and to prevent remelting of areas other than the end portion of the slab.

[0179] When solidification of the slab S is complete up to its end, as shown in FIG. 3(d), the moving unit 180 is operated to push the slab S, which is placed vertically on the support unit 171, toward the rotating unit 190. As a result, the slab S on the support unit 171 is transported to the rotating unit 190. At this time, the side of the slab S is supported by the first table 191-1 of the rotating unit 190, and the bottom surface of the slab is supported by the second table 191-2. Next, the rotating unit 190 is rotated, i.e., tilted (S240). More specifically, the rotating unit 190 is rotated so that the first table 191-1 is parallel to the ground and the second table 191-2 is horizontal to the ground. As a result, the slab S is rotated so that it is placed horizontally relative to the ground. The slab placed horizontally on the rotating unit 190 is transported to the subsequent process.

[0180] Once the slab S is produced, it is heated using the first heating device 200 (S300). At this time, the temperature inside the furnace in which the slab S is charged is gradually increased to 1140°C to 1240°C (target temperature) to heat the slab. More specifically, the slab S is first heated to 250°C to 350°C (first temperature) and held there for 3 to 5 hours. Thereafter, the slab S is heated to 450°C to 550°C (second temperature) and held there for 5 to 7 hours. Next, the slab S is heated to 650°C to 750°C (third temperature) and held at the third temperature for 3 to 5 hours. Next, the slab S is heated to the target temperature of 1100°C to 1250°C (fourth temperature) and held there for 14 to 18 hours. In this way, when heating the slab S to 1100°C to 1250°C (fourth temperature), by gradually increasing the temperature from the first to fourth temperatures as described above, it is possible to suppress or prevent the occurrence of thermal cracks in the slab containing a large amount of chromium (Cr) of 4.5 wt% or more.

[0181] After the step of heating the slab S in the first heating device 200 is completed, the slab is forged using the forging device 300 to manufacture a rolling roll (S400). At this time, the thickness reduction of the slab S per pass is set to 250 mm to 350 mm, and this is repeated multiple times to manufacture the rolling roll 141.

[0182] Once the mill roll R is manufactured, the mill roll 141 is heated using the second heating device 400 to remove hydrogen (H2). At this time, the mill roll 141 is heated at a temperature of 200°C to 400°C for 48 hours or more. As a result, the hydrogen content in the mill roll 141 can be adjusted to 0.0002 wt% or less (0 wt or more).

[0183] Table 1 is an evaluation table for the mill roll manufactured by the method according to the embodiment of the present invention. For quality evaluation, a part of the mill roll manufactured by the method according to the embodiment was cut and used as a test piece.

[0184] Ultrasonic testing (UT) quality evaluation is a quality evaluation method used to detect internal defects such as pores, cracks, etc. In other words, it is an evaluation method that transmits ultrasonic waves into the inside of a specimen and detects the presence and amount of pores and cracks inside the specimen using the amount of energy of the ultrasonic waves reflected from discontinuous parts inside the specimen and the propagation time of the ultrasonic waves.

[0185] Macro quality evaluation is a quality evaluation method to determine the presence and amount of coarse structures. For the evaluation, the specimen was etched using corrosion, and then the internal structure of the specimen was observed using an optical microscope.

[0186] Inclusion quality evaluation is a quality evaluation method for determining the amount and size of inclusions present in a specimen. For the evaluation, a portion of the specimen was polished and then the size and amount of inclusions were detected using an optical microscope.

[0187] The hardness is the hardness of the test piece and was measured using a Brinell hardness tester.

[0188] [Table 1]

[0189] As shown in Table 1, the specimens manufactured by the method according to the embodiment are all at pass levels in terms of UT quality, macro quality, inclusion quality, and hardness quality. That is, the evaluation results of the roll manufactured by the method according to the embodiment are all pass in terms of UT quality, macro quality, inclusion quality, and hardness quality. This shows that by manufacturing a roll by the method according to the embodiment, it is possible to manufacture a roll having less pores, cracks, and inclusions, a uniform structure, and high hardness.

[0190] FIGS. 5(a) and (b) show the results of cutting a cross section of a rolling roll, etching the cross section, and checking whether or not coarse segregation has occurred.

[0191] 5(a) is a cross section of the manufactured roll, and FIG. 5(b) is a cross section of the roll manufactured by the method according to the comparative example. The roll manufactured by the method according to the comparative example is a roll manufactured through an electroslag remelting (ESR) method.

[0192] Comparing (a) and (b) of Figure 5, no coarse segregation occurred in either the rolling roll or the comparative rolling roll. That is, the rolling roll has a uniform structure equivalent to that of the comparative rolling roll. This shows that segregation can be sufficiently suppressed even when the rolling roll is manufactured without using the conventional electroslag remelting (ESR) method. That is, even when the slab is cast using the method of fluidizing the molten steel using first and second magnetic field generators during the casting process, segregation can be suppressed to a level equivalent to that of the conventional electroslag remelting (ESR) method.

[0193] Furthermore, compared to the conventional electroslag remelting (ESR) method, the method of suppressing segregation using the first and second magnetic field generators during the casting process is simpler and requires less time. That is, it has the advantage of being able to suppress segregation to the same level as the conventional method, while shortening the process steps required for segregation generation and shortening the process time. [Industrial Applicability]

[0194] According to the present invention, high chromium (Cr) molten steel can be produced using a converter used for making steel of other steel types without contaminating the converter used for making steel of other steel types. Furthermore, when raising the temperature of the high chromium (Cr) molten steel or performing degassing by venting gases, the temperature of the molten steel can be effectively raised, thereby improving degassing efficiency. [Explanation of symbols]

[0195] 11 Converter 20 ladle 30 Temperature control device 31 Electrode rod 32 Cover 40 RH equipment, vacuum equipment, vacuum oxygen decarburization (VOD) equipment 41 Vessel 42a, 42b Reflux tube 43 Lance 100 Casting equipment, vertical casting equipment 120 Mold 140 Cooling section 141 Multiple rolls, rolling rolls 150a First magnetic field generating unit 150b Second magnetic field generating unit 160 Heating section 171 Support part 172 Drive unit 172-1 Power source 172-2 Driving member 180 Mobile Unit 190 Rotating part 191 Rotating Table 191-1 First Table 191-2 Second Table 191-3 Multiple Rollers 192 Rotating members 200 First heating device 300 Forging equipment 400 Second heating device (S100) The process of preparing molten steel (S110) Converter treatment process (S111) The process of adding ferroalloy (S112) Oxygen blowing process (S113) Deoxidation process (S114) The process of reducing chromium oxide to chromium (Cr) (S120) The process of tapping steel into a ladle (S130) Temperature adjustment process, degassing process (S140) Degassing process (S200) The process of solidifying molten steel to produce cast slabs (S300) Heating the slab (S400) The process of forging heated cast pieces to produce rolling mill rolls (S500) Heating the rolling rolls (LF) Ladle refining furnace (M) Molten steel (VTD) Vacuum Tank Degasser

Claims

1. A method for producing high chromium (Cr) molten steel having a chromium (Cr) content of 4.5 wt% to 5.5 wt%, comprising: A process of charging molten steel into a converter used in a stainless steel making process; adding chromium (Cr)-containing ferrochromium alloy into the converter so that the chromium (Cr) content in the molten steel is 4.5 wt% to 5.5 wt%; A method for producing high chromium (Cr) molten steel, comprising:

2. an oxygen blowing process in which oxygen is blown into the converter into which the chromium ferroalloy is charged to remove carbon (C) from the molten steel; a step of adding a deoxidizer to the converter to remove oxygen (O) from the molten steel; a step of adding a reducing agent to the converter to reduce chromium oxide contained in slag floating on the surface of the molten steel to chromium (Cr); The method for producing high chromium (Cr) molten steel according to claim 1, comprising:

3. tapping the molten steel from the converter into a ladle; a temperature adjusting step of heating molten steel received in a ladle using a ladle refining furnace (LF), which is a heating device used in a carbon steelmaking process; The method for producing high chromium (Cr) molten steel according to claim 1, comprising:

4. 4. The method for producing high chromium (Cr) molten steel according to claim 3, wherein the temperature of the molten steel is adjusted to 1560°C to 1600°C in the temperature adjusting step.

5. The hydrogen (H) in the molten steel received in the ladle is removed by using a reflux vacuum degassing (RH) device, which is a vacuum device used in the carbon steel manufacturing process. ２ ) and nitrogen (N ２ ) degassing process, 4. The method for producing high chromium (Cr) molten steel according to claim 3, wherein the degassing process is performed after the temperature adjustment process is completed.

6. The method for producing high chromium (Cr) molten steel according to any one of claims 3 to 5, characterized in that when the molten steel from the converter is tapped into a ladle, the ladle is used in a steelmaking process for carbon steel.

7. preparing molten steel containing 4.5 wt% to 5.5 wt% chromium (Cr); a casting process in which molten steel is supplied to a mold of a casting device and the molten steel is solidified inside the mold to produce a cast containing unsolidified molten steel; a step of withdrawing the cast piece produced in the casting step to below the mold; a solidification step in which a magnetic field is applied to the slab drawn below the mold to solidify the unsolidified molten steel contained in the slab while causing it to flow, thereby producing a slab; A method for producing a slab, comprising:

8. 8. The method for producing a slab according to claim 7, wherein the solidification step includes a step of heating an end portion of the slab drawn below the mold.

9. 8. The method for producing a slab according to claim 7, wherein the casting step includes a step of applying a magnetic field to the mold to cause the molten steel inside the mold to flow.

10. 8. The method for producing a slab according to claim 7, wherein the slab is withdrawn below the mold at a speed of 0.04 m / min or less.

11. The solidification step includes a step of injecting cooling water onto the cast piece drawn below the mold, 8. The method for producing a cast slab according to claim 7, wherein the cooling water is sprayed onto the cast slab so that the surface temperature of the cast slab becomes 800°C to 900°C.

12. In the process of withdrawing, the cast piece is withdrawn from the mold so as to be perpendicular to the ground; The method for producing a slab according to any one of claims 7 to 11, wherein the slab is solidified in a state where it is positioned perpendicular to the ground during the solidification process.

13. A step of heating a slab produced by the method for producing a slab according to any one of claims 7 to 11; forging the heated billet into the shape of a rolling mill roll; The rolls manufactured in the forming process are heated to extract hydrogen (H ２ ) and Including, The method for manufacturing a rolling roll, wherein the step of heating the slab includes a step of heating the slab by raising the temperature of the slab to a target temperature in a plurality of stages.

14. The process of heating the slab to the target temperature includes: heating the strand to a first temperature of 250°C to 350°C; heating the strand to a second temperature of 450°C to 550°C; heating the strand to a third temperature of 650°C to 750°C; heating the slab to the target temperature of 1100°C to 1250°C; The method for manufacturing a mill roll according to claim 13, comprising:

15. When the cast piece is heated to the first to third temperatures and the target temperature, Holding the slab at the first temperature for 3 to 5 hours; Holding the slab at the second temperature for 5 to 7 hours; Holding the slab at the third temperature for 3 to 5 hours; The method for manufacturing a rolling roll according to claim 14, wherein the cast strip is held at the target temperature for 14 to 18 hours.

16. When forging the cast piece, the cast piece heated to a temperature of 1100°C to 1250°C is forged by reduction; The method for manufacturing a rolling roll according to claim 15, wherein the thickness of the slab is reduced by 250 mm to 350 mm each time the slab is reduced using the reduction device.

17. The hydrogen (H ２ 14. The method for manufacturing a mill roll according to claim 13, further comprising the step of heating the mill roll to a temperature of 200°C to 400°C when removing the sintered material.

18. The method for manufacturing a mill roll according to claim 17, wherein the mill roll is heated to a temperature of 200° C. to 400° C. for 48 hours or more.

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